Re: [RFC PATCH v2 6/6] x86/entry/pti: don't switch PGD on when pti_disable is set

[Willy Tarreau]

On Thu, Jan 11, 2018 at 01:28:18PM -0800, Linus Torvalds wrote:
> The traditional fast system call to test is getppid().
> 
> write() goes through a lot more code.

Just tried getppid() now, it's relatively similar (slightly slower than
write(-1) though, maybe that one aborts very early) :

  PTI=on          : 920ms for 3 million calls
  PTI=off (prctl) : 230ms for 3 million calls
  PTI=off (boot)  : 215ms for 3 million calls

The small difference between the last two very likely comes from the few
instructions avoided thanks to the alternatives when pti=off is used at
boot.

So yes here it's trivial to tell if it's on or off :-)

Willy